1. Field of the Invention
The present invention concerns multifocal artificial ocular lenses, by which is meant both rigid material artificial ocular lenses and flexible material artificial ocular lenses.
In the present context the expression artificial ocular lenses means contact lenses, intra-ocular implants or intracorneal lenses.
2. Description of the Prior Art
Multifocal contact lenses have at least two corrective areas, namely a near vision area adapted to correct near vision and a distance vision area adapted to correct distance vision.
One such multifocal contact lens has two concentric corrective areas, for example, disposed so that the near vision area is at the center of the lens and the distance vision area is at its periphery, in which case the lens is usually called a "central NV" lens.
Alternatively, the distance vision area is at the center and the near vision area is at the periphery, in which case the lens is usually called a "central DV" lens.
The vision performance of most multifocal contact lenses of this type is strongly dependent on the pupil diameter of the user.
The pupil of the eye exhibits a natural reflex reaction to two types of stimulation: its diameter decreases as the distance between it and the observed object is reduced and/or when the luminance of that object increases.
Variations in the pupil diameter related to the distance of the object can be exploited by appropriate multifocal optics, and in practise they are operative in the appropriate sense with central NV lenses.
With these lenses, the constriction of the pupil that accompanies near vision automatically leads to at least partial selection of the appropriate corrective area, and the same goes for the dilation of the pupil that accompanies distance vision.
In both cases the result of the reaction of the pupil to the observation distance is an improvement in the contrast in the image obtained on the retina.
On the other hand, the variations in the pupil diameter associated with the luminance of the observed object, and therefore in practise with its illumination, have undesirable effects.
With a central NV lens, for example, strong illumination results in constriction of the pupil which, by the resulting selection of the effective corrective area, favors near vision correction to the detriment of distance vision correction, even if the observed object is at a great distance.
Similarly, with a central DV lens strong illumination favors the correction of near vision to the detriment of distance vision even if the observed object is at a short distance, as may be the case for reading in a well lit place, for example.
In both cases the change from low luminance to high luminance is therefore prejudicial to the stability of visual performance, or more generally to the quality of optical performance, to the detriment of the visual comfort of the user.
A general object of the present invention is an arrangement that minimizes, if not eliminates, this drawback.